The RCAS/RCAN retroviral vectors are in wide use as tools for studying development and oncogenesis in the avian system. The creation of transgenic mice that express a specific receptor for these vectors has significantly increased the flexibility of this system. We have developed lines of transgenic mice that express the receptor under the control of a tissue-specific promoter; this makes it possible to direct the vector to particular cells or tissues. This system can be used for lineage analysis; it will simplify studies of the effects of oncogenes and morphogenesis in vivo; in addition, the ability to make defined mutations in the mouse means that this can be an efficient model system to evaluate gene therapy strategies. We are particularly interested in the applications of this system to study the effects of aging skeletal muscle; however, we would point out that it should be relatively simple to use the RCAS/RCAN vectors and appropriate strains of subgroup A receptor mice both to study and test therapeutic strategies in a variety of other tissues.